Neisseria gonorrhoeae (Gc) is an obligate human pathogen that is the causative agent of the sexually transmitted disease gonorrhea. All freshly isolated Gc express pili on their cell surfaces; and the expression of the pilus is required for infection in human volunteers. The most unique characteristic of Gc pili is the large number of possible pilus antigenic types that a single organism can produce. This antigenic variation process occurs by recombination between one of several silent pilin gene copies and the singular expressed gene (that encodes the major subunit of the pilus, pilin), resulting in multiple sequence changes in the expressed gene and protein. This system of pilin antigenic variation provides a large mosaic of antigenic types in a Gc population, and allows continual reinfection of the high risk portion of the human population that transmits Gc into the general population. Much of our understanding about the mechanisms that produce and control pilin antigenic variation has been obtained by determining the types of sequence changes that occur when an antigenic switch occurs. For each antigenic switch, a portion of one of 19 silent pilin gene copies is transferred to the pilE locus in a nonreciprocal fashion. The investigators have isolated a number of cis and trans-acting mutation that disrupt the process of pilin antigenic variation and predict molecular mechanisms that promote the high frequency transfer of variable pilin sequences. They will continue to isolate and characterize genes, proteins, and cis-acting DNA sequences that are required for pilin antigenic variation to test between different hypotheses about how pilin antigenic variation is mediated. They will also explore the possibility that pilin antigenic variation is regulated during human infection. Through these investigations, this project will continue to define the molecular mechanism used to allow these high frequency recombination reactions that are of critical importance to the pathogenesis of gonorrhea.